40 Mr. O. Thomas on new Mammals from 



individual are at the same stage of development. Gastrula- 

 tion was not observed in the living material, but sections show 

 a stage in which central cells lie clearly marked off from a 

 surrounding ring of smaller surface-cells, and the probability 

 is that these endodermal cells were formed by delamination. 

 When the spherical mass is beginning to elongate into the 

 ellipsoidal form of the planula a large migration of cells from 

 the surface into the centre takes place, not limited to one pole. 

 The relation of the central cells earlier seen and of these in- 

 wandering cells to the definitive endoderm u'^as not made out. 

 The young are set free from the parent as ciliated planul^e, 

 having pigment spots on the posterior end. The planulae 

 swim about actively on the surface for a day or two, their 

 motion being both translation and rotation. Then they 

 gradually lose the forward motion and rotate on their own 

 axis in one spot. At this point they settle down and the 

 pigment spots which were on the posterior surface are found 

 to migrate into the interior. In a day or two they bud out 

 two tentacles, and shortly afterwards two more, though young 

 hydras with three and tive tentacles were by no means 

 uncommon. In this condition they lived for three weeks in 

 the aquaria without undergoing further development. 



Search for scyphistomas in the region where the jellyfish 

 were found was fruitless. The youngest specimens taken 

 in the tow were of practically the adult structure, so that the 

 important intermediate stages remain unknown. The 

 youngest jellyfish were interesting, however, in that only the 

 middle tentacle in each group was developed, and that the 

 sensory clubs were situated almost on the margin, and were 

 not yet enclosed in niches. 



VII. — On new Mammals from Western Mexico and Lower 

 California. By Oldfield Thomas. 



The British Museum has received from Mr. W. W. Price 

 two collections of mammals — one from Sinaloa, W. Mexico, 

 obtained by Mr. P. O. Simons, and the other from the Cape 

 region of Lower California, collected by Mr. D. Coolidge. 



In the former collection occur the remarkable pigmy skunk 

 with uninterrupted dorsal lines [Spilogale pygmcea*) exhibited 

 recently at theZoological Society, and the jaguarondi described 

 below. The remainder of the collection consists mainly of 



* Diagnosed in Abstract of Meeting of Nov. 30. Published Dec. 1, 

 1897. 



